Systems for refractive index measurement of a sample using the critical angle are well known in the art, as are the principles of physics underlying the measurement of critical angle to determine refractive index of a medium. When light traveling from a high index medium is incident upon an interface between the high index medium and another medium having a lower refractive index at angles of incidence larger than a critical angle of incidence, total internal reflection may be observed. The critical angle is a function of the refractive index of both media. However, if the refractive index of one medium is known, the refractive index of the other may be determined from a measurement of the critical angle θc using the well-known formula:
      sin    ⁢                  ⁢          θ      c        =                    n        2                    n        1              .  
Where n1 is the refractive index of the high index medium and n2 is the refractive index for the low index medium. By convention, the critical angle of incidence is measured with respect to a line perpendicular to the interface between the two media.
U.S. Pat. No. 6,097,479 describes a sensor for making critical angle measurements in which a light source and photodetector array are encapsulated in a light transmissive housing that acts as the high index medium. The housing forms a prism having one face in contact with a sample, which acts as the low index medium. Light from the light source is incident on an interface between the sample and the prism over a range of incident angles. A portion of the light that is incident on the interface at angles greater than a critical angle undergoes total internal reflection and is detected by the photodetector array. Different parts of the photodetector array are therefore illuminated by the totally internally reflected light depending on the critical angle, which depends on the index of refraction of the prism and of the sample. The pattern of illumination of the photodetector array can be analyzed to determine the index of refraction of the sample.
It is within this context that embodiments of the present invention arise.